Cyclic carbonates, cyclic esters of carbonic acid, have been prepared by many well-known methods. The reaction of ethylene glycol with phosgene to produce ethylene carbonate, the simplest of the cyclic esters, is more than a century old (J. prakt. Chem. 28 (2) 439 (1883)). Because of the extreme toxicity and corrosivity of phosgene, alternative routes to cyclic carbonates have been explored.
U.S. Pat. No. 1,907,891 teaches the preparation of cyclic carbonates from sodium bicarbonate and alkylene halohydrins.
U.S. Pat. Nos. 4,181,676 and 3,642,858, and German Patent 2,615,665 teach base-catalyzed preparation of acyclic dialkyl carbonates from cyclic carbonates. Japanese Kokai 88-238,043 teaches the same reaction catalyzed by anion exchange resins having quaternary ammonium groups.
Synthetic approaches to commercially interesting cyclic carbonates, especially ethylene carbonate and propylene carbonate, have focused on the reaction between an epoxide and carbon dioxide, a reaction first disclosed in German Patent 740,366 (1943). The process has been improved substantially by the use of various catalysts, including tetraalkylammonium halides (U.S. Pat. No. 2,773,070) and quaternary ammonium bases (U.S. Pat. No. 2,667,497). Some of this work is summarized in W. J. Peppel, Ind. Eng. Chem. 50 (1958) 767. The epoxide/CO.sub.2 reaction has been catalyzed by other additives, including anion exchange resins (U.S. Pat. No. 4,233,221), amines (U.S. Pat. No. 2,773,881), and quaternary ammonium hydroxides, carbonates or bicarbonates (U.S. Pat. No. 2,873,282).
Recent cyclic carbonate syntheses include reaction of a diol with an alkyl trichloroacetate in the presence of base (U.S. Pat. No. 4,344,881), thermal decomposition of halogenated aliphatic carbonates (U.S. Pat. No. 4,332,729), reaction of a 1,2-diol with chlorosulfonyl isocyanate (Synth. Commun. 18 (1988) 2295), reaction of tertiary 1,2-diols with acetic anhydride and 4-dimethylaminopyridine (J. Org. Chem. 49 (1984) 3974), and reaction of a diol with di-2-pyridyl carbonate (Heterocycles 24 (1986) 1625).
Transesterification of acyclic diesters of carbonic acid with glycols in the presence of an alkaline catalyst to give cyclic carbonates is described by M. Morgan and L. Cretcher (J. Am. Chem. Soc. 68 (1946) 781). Preparation of ethylene carbonate from ethylene glycol and diethyl carbonate in the presence of potassium carbonate is described. U S. Pat. No. 2,441,298 teaches the preparation of ethylene carbonate from ethylene glycol and diethyl carbonate using metallic sodium as the catalyst. The use of sodium methoxide as the alkaline catalyst for making 6-membered cyclic carbonates is illustrated by Sarel and Pohoryles (J. Am. Chem. Soc. 80 (1958) 4596). Dialkyltin oxides are described as transesterification catalysts in U.S. Pat. No. 3,663,569. Other compounds known to catalyze the transesterification reaction between acyclic diesters of carbonic acid and diols, typically bases and transition metal compounds, are outlined in U.S. Pat. Nos. 4,440,937 and 3,426,042. These include, among other catalysts, oxides, hydroxides, alcoholates, carboxylates, and carbonates of sodium, potassium, aluminum, thallium, and lead, as well as various titanium compounds, metal chelates, and manganese salts.
A disadvantage of basic and transition metal transesterification catalysts is that they often catalyze the polymerization of cyclic carbonates, especially cyclic carbonates having 6-membered rings. Consequently, the use of these catalysts often results in product mixtures that contain unwanted polycarbonate polymers in addition to the desired cyclic carbonates. A transesterification process that gives cyclic carbonates free from polycarbonate by-products is needed.